Block copolymers comprising one or more alkenylarene polymer block and one or more olefinic polymer block are generally known as thermoplastic elastomers (TPE's). The block copolymers are elastomeric in the sense that they typically have a three-dimensional, entangled (alternatively known as “physically crosslinked”) structure below the glass transition temperature (Tg) of the stryenic block such that they exhibit elastic memories in response to external forces. The block copolymers are thermoplastic in the sense that they can be softened or melted above the glass or crystalline transition temperature of the alkenylarene block, processed, and cooled/solidified several times with little or no change in physical properties (assuming a minimum of oxidative degradation).
These block copolymers are known to have high strength and elasticity at ambient temperatures. The high strength and elasticity of these block copolymers are due to the microphase separated network structure wherein the olefinic blocks and the alkenylarene blocks separate from structurally dissimilar blocks and entangle with structurally similar blocks to form separate domains. The olefinic blocks typically have a glass transition temperature below ambient temperature, thus, they are relatively free to move about and form the soft, rubbery phase at or above ambient temperature. In contrast, the alkenylarene blocks have a glass and/or crystalline transition temperature above ambient temperature, thus, they are relatively immobilized in the entangled state and form the hard phase. However, at body temperature, the copolymers may begin to lose their mechanical properties after some time. The deterioration of properties appears to be associated with the copolymer movements, especially the movements of the alkenylarene blocks. At body temperature, sometimes accompanied with tension or load, the previously immobile alkenylarene blocks begin to slip pass neighboring alkenylarene blocks. Since the alkenylarene blocks form the hard phases, which are primarily responsible for the mechanical properties, such motions of the alkenylarene blocks adversely effect the mechanical properties of the copolymer.
Plasticizers or processing oils are often added to the block copolymers to lower the viscosity and improve the processability of the block copolymers. Other polymers may also be added to compatibilize the blends and/or improve the mechanical properties. Blends comprising block copolymers are described in U.S. Pat. Nos. 3,562,356 (Nyberg et al.); U.S. Pat. No. 4,704,110 (Raykovitz et al.); U.S. Pat. No. 4,578,302 (Schmidt et al.); U.S. Pat. No. 5,503,919 (Litchholt, et al.); U.S. Pat. No. 5,540,983 (Maris et al.); U.S. Pat. No. 6,117,176 (Chen); and U.S. Pat. No. 6,187,425 (Bell et al.).
However, the addition of the plasticizers and/or processing oils lower the strength and elastic properties of the block copolymer compositions.
Therefore, it is desirable to provide a novel material that lowers the viscosity and improves the processability of block copolymer compositions without substantially compromising their mechanical properties.
It is also desirable to provide a novel material that exhibits a phase change as the temperature is raised and/or lowered such that the novel material effects very sharp changes in the characteristics (e.g., viscosity) of the block copolymer compositions at or around the phase change temperature of the novel material.
It is further desirable that the phase change temperature of the novel material can be controlled by its molecular characteristics, such as the monomeric structure, the molecular weight, the aromatic and aliphatic carbon content in the backbone, and the like.
Moreover, it is desirable that the viscosity of the phase change solvent and of its block copolymer blends can be varied over a broad range to achieve the suitable viscosity for different fabricating processes, such as extrusion, injection molding, melt spinning, blow molding, spraying, printing, coating, and the like.